In recent years, a motor has often employed a method in which a motor is driven by an inverter using a pulse width modulation system (to be referred to as a PWM system hereinafter). In the inverter driving of the PWM system, since a neutral-point potential of a winding is not zero, a potential difference (to be referred to as a shaft voltage hereinafter) is generated between an outer ring and an inner ring of a bearing. The shaft voltage includes a high-frequency signal obtained by switching. When the shaft voltage reaches a breakdown voltage of an oil film in the bearing, a minute electric current flows in the bearing to cause electric corrosion in the bearing. When the electric corrosion progresses, a wavelike abrasion phenomenon may occur on a bearing inner ring, a bearing outer ring, or bearing balls to generate abnormal sound, and the abnormal sound is one main factor of drawback in the motor.
Conventionally, in order to suppress electric corrosion, the following measure is conceived.
(1) The bearing inner ring and the bearing outer ring are electrically conductive to each other.
(2) The bearing inner ring and the bearing outer ring are electrically insulated from each other.
(3) A shaft voltage is reduced.
As a concrete method for the (1), a method of giving conductivity to a lubricant agent of the bearing is given. However, a conductive lubricant agent is deteriorated in conductivity with elapsed time, and lacks in sliding reliability. A method of arranging a brush on a rotating shaft to set a conductive state may be conceived. However, this method disadvantageously requires brush abrasion powder or a space.
As a concrete method of the (2), a method of changing iron balls in the bearing with nonconducting ceramic balls is given. This method highly effectively suppresses electric corrosion but disadvantageously has high cost. The method cannot be employed in a general-purpose motor.
As a concrete method of the (3), a method of electrically short-circuiting a stator iron core to a conductive metal bracket to change a capacitance to reduce shaft voltage is conventionally known (for example, see Patent Document 1).
An impedance obtained when an electric capacity and a resistor are connected in parallel with each other is expressed by a relational expression: Z=1/jωC+R. In this expression, Z denotes an impedance, j denotes an imaginary number, ω denotes an angular frequency, C denotes a capacitance, and R denotes a resistance. As is apparent from expression, when the capacitance increases or when the resistance decreases, the impedance decreases. In contrast to this, when the capacitance decreases or when the resistance increases, the impedance increases.
In Patent Document 1, the stator iron core and the bracket are short-circuited to decrease an impedance on the stator side. In this manner, the bearing is suppressed from being electrically corroded.
More specifically, in general, a motor that is used in a washing machine, a dishwasher, or the like near water and that may electrocute a person needs not only to have a charging unit insulated (basic insulation) but also to be additionally independently insulated (to be referred to as additional insulation hereinafter). On the other hand, since a motor that is used in an indoor unit or an outdoor unit of an air conditioner, a water heater, an air purifier, or the like in addition to a washing machine, a dishwasher, or the like may not electrocute a person, the motor does not require additional insulation. Therefore, since the motor that is used in an indoor unit or an outdoor unit of an air conditioner, a water heater, an air purifier, or the like does not have an insulated rotor, an impedance on the rotor side (bearing inner ring side) is at a low level. In contrast to this, since the stator side (bearing outer ring side) has an insulated structure, an impedance is at a high level. In this case, a potential on the bearing inner ring side is high, and a potential on the bearing outer ring is low. For this reason, an imbalanced state occurs, a high shaft voltage is consequently generated. The high shaft voltage may cause electric corrosion in the bearing.
In order to avoid the state, Patent Document 1 employs a method that short-circuits the stator iron core and the bracket to each other to eliminate a capacitance component therebetween and, as described above, decreases the impedance on the stator side (bearing outer ring side) to approximate the impedance to the impedance on the rotor side (bearing inner ring side).
However, the conventional method as described in Patent Document 1 has the following problem. More specifically, the conventional method is a method that changes an impedance on a stator side to keep a potential balance between the bearing inner ring and the bearing outer ring to try to suppress electric corrosion. In this method, when an impedance balance was broken by a using environment of the motor or a variation in assembling accuracy of the stator and the rotor, on the contrary, a case in which the shaft voltage became high to easily cause electric corrosion was considered as a possible case.    Patent Document 1: Unexamined Japanese Patent Publication No. 2007-159302